1. Field of the Invention
The present invention is directed to underground heat exchange systems; to apparatus and methods for installing a loop of pipe in a hole in the earth; and to thermally conductive filler materials for emplacement between the pipe loop and the interior hole wall.
2. Description of Related Art
The prior art discloses the use of a common grout, typically a bentonite clay mixture, for use as a thermally conductive material between a pipe loop of an underground heat exchange system and the interior of a hole in which the loop is positioned.
The cost for the installation of certain prior art vertical ground source heat loops can account for nearly half of the total cost incurred in the installation of a geothermal heat pump system. In certain aspects, the heat loop installation consists of drilling a vertical well, placing a thermoplastic pipe xe2x80x9cloopxe2x80x9d in the wellbore, and filling the annular space between the loop and the well wall with a thermally conductive material. Several problems can arise in such methods: loop xe2x80x9cinsertionxe2x80x9d difficulties caused by loop buoyancy; thermal conductivity and installation difficulties of borehole backfill materials; and environmental concerns.
Although the drilling of the hole may be relatively easy, certain problems may be encountered in inserting the loop. In most application, the xe2x80x9cloopxe2x80x9d consists of a relatively lightweight thermoplastic pipe. One material of choice is polyethylene, with a specific gravity of approximately 0.955 gms/cc3 lighter than water. This pipe material also has relatively poor stiffness. As a driller attempts to push the pipe down into a mud filled well, the natural buoyancy of the pipe resists these efforts. Since the pipe has poor longitudinal stiffness, the pipe tends to bend or curl inside the well, creating additional frictional drag against the well wall until the pipe can be pushed no deeper. Even when the pipe is filled with water, the loop still maintains considerable buoyancy because the drilled hole is filled with dense drilling mud. In soft geological formations, the driller must mix heavy drilling mud in order to transport drilled cuttings and to stabilize the hole. In most cases, bentonite clay is added to the drill fluid to reduce friction, prevent loss of circulation, and suspend solids. However, the result is residual heavy drilling fluid in the well, creating a very dense slurry of clay, sand, rock, etc.xe2x80x94with resulting high buoyancy and low surface tension. In order to overcome the buoyancy problems, the driller usually attaches a heavy steel bar to the leading end of the heat loop and xe2x80x9cpullsxe2x80x9d the loop to the bottom of the well. Once the loop hits bottom, the driller 1) remotely secure the loop in the hole to prevent it from floating up, 2) remotely detaches the steel bar from the loop, 3) and recovers the bar from the well, usually by a cable winch. The considerable efforts made to overcome buoyancy of the loop caused by the density of the drill fluid are time consuming, expensive, and hazardous to the integrity of the loop.
The ability of the loop to transfer heat is directly related to the efficient operation of a geothermal heat pump system. Great expense and many tests and studies have been performed in a search for an optimum thermally conductive material that can be economically placed in the annular space between the loop pipe and the wellbore wall. An ideal material would be: at least as thermally conductive as the native earth; be easily and reliably placed in the borehole; maintain conductivity xe2x80x9clong termxe2x80x9d, and be inexpensive.
In some applications, particularly those where a loop is installed below a water table, no xe2x80x9cbore backfillxe2x80x9d material is placed in the well. The water is extremely thermally conductive (if it remains in the bore long term). The concern is that over time, the water table may drop, leaving an insulating airspace between the loop and the borehole. This insulating airspace results in xe2x80x9chot loopsxe2x80x9d, and the heat pump system either works poorly or not at all.
Many conductive high solids and cementitious xe2x80x9cgroutsxe2x80x9d have been developed which include a mixture of water and a relatively conductive solid such as silica sand or fly ash. In order to transport the heavy solids, a viscosity enhancer such a bentonite clay is added. Also, friction reducers and xe2x80x9csuper-plasticizersxe2x80x9d, like sulfonated naphthalene, are added to make the slurry pumpable. Although these grouts seem to work well in testing, there is great resistance from the field installers. The mixture can be expensive to handle, difficult to mix, even more difficult to pump, and extremely rough on equipment because of the abrasion. In addition, placing of the material in the borehole is difficult to control and monitor. If grouting is attempted from the surface, xe2x80x9cbridgingxe2x80x9d can occur resulting in a partially filled hole. If grouting from the bottom of the hole, xe2x80x9cchannelingxe2x80x9d can occur, again resulting in a partially grouted hole. When the water subsides in the borehole, this leaves air spaces, thus insulating the loop and reducing the efficiency of the system. An additional disadvantage is that the voids from channeling as well as the interstitial spacing between the individual sand (or other solid particles) create permeability. Permeability that creates a vertical communication path by which the ground water system could be contaminated by surface spills is an environmental concern.
The prior art cementitious grouts, developed to overcome the permeability objection, have some unique problems of their own. In addition to all of the handling, mixing, pumping, abrasion, and conductivity problems of the High Solids Thermally Conductive Grouts, the xe2x80x9cheat of hydrationxe2x80x9d generated when cement cures, causes grout to xe2x80x9cshrinkxe2x80x9d away from the polyethylene pipe. As heat is generated, the polyethylene pipe, having a very high coefficient of expansion expands. Once the cement cures and cools, the polyethylene contracts and actually pulls away from the grout. Although the permeability of the cured grout itself is very low, a flow path may now exist between the polyethylene pipe and the groutxe2x80x94again insulating the loop and threatening the environment. Because of its inherent structure, polyethylene is a very high molecular weight wax or paraffin, and does not bond well with anything, even under laboratory controlled conditions. Attempts to control grout shrinkage and cement to polyethylene bonding in the field were proven unsuccessful.
Drilling xe2x80x9cpolymersxe2x80x9d have existed for years. They have been used in the oil well drilling industry as an alternative to bentonite clays to provide solids transport, solids suspension, friction reduction, and displacement efficiency.
The prior art discloses a variety of systems and apparatuses for installing ground heat exchange pipe loops in a wellbore, including a system in which a wellbore is drilled, e.g. a vertical hole four to four-and-a half inches in diameter to a depth of about 250 feet, and a single piece of polyethylene pipe attached to a sinker bar is introduced into the hole and then pulled out of the hole manually while grout is introduced into the hole. A pipe loop (polyethylene) is pushed to the bottom of the hole by a wire-line retrievable sinker bar. With the sinker bar removed, a series of screwed together 2 inch PVC tremmie pipes is lowered to the bottom of the hole and grout mixed at the surface is pumped into the tremmie pipe. As each batch is pumped into the hole the tremmie pipe string is raised and one 20 foot section of pipe is removed from the hole. After grouting is completed and the tremmie pipe is removed, the rig is moved to another drilling position, e.g. at least 15 feet away. When all of the pipe loops have been installed (e.g. one loop for each ton of heating and cooling equipment), the drill rig is removed from the site. A trench (e.g. about four feet deep) is then dug to contain pipes that interconnect all of the pipe loops and a connecting pipe is laid into the trench, heat fused to each of the vertical pipe loops, and pressure tested and buried to serve as a circulating manifold carrying water between the earth and a heat pump located within an adjacent building. The trenching and manifolding of the surface pipe typically takes as much time as the wellbore drilling and pipe installation.
The prior art discloses numerous in-ground heat exchanger systems (e.g. see U.S. Pat. Nos. 5,244,037; 5,261,251); and grouting systems (see, e.g. U.S. Pat. No. 5,435,387).
The present invention discloses, in certain aspects, a wellbore heat loop system including a heat loop wellbore in the earth extending from an earth surface down into the earth to a bottom of the wellbore, a heat loop disposed in the heat loop wellbore and extending down to a position near the bottom thereof, the heat loop comprised of heat loop pipe, filler material around the heat loop in the wellbore, the filler material including a gel with an amount of water, and an amount of a gel material mixed with the water. In certain aspects the filler material has a thermal conductivity of at least 1.2 or at least 1.4 and includes gelling polymer, solids and water present, by weight, in the ranges of polymerxe2x80x94about 0.3% to about 5%, waterxe2x80x94about 25% to about 50%, and solidsxe2x80x94about 50% to about 80%.
The present invention also discloses, in certain embodiments, a bottom member for an earth bore heat loop system, the bottom member having a body, a first bore through the body extending from a first opening of the body to a second opening of the body, the first opening and the second opening each sized and configured for receipt therein of an end of heat loop pipe, a second bore having at least one opening on the body, the second bore sized and configured for securement thereat of an end of coil tubing; and at least one fluid exit port in fluid communication with the second bore for jetting fluid from the bottom member.
The present invention, in certain aspects, discloses a ground heat exchange system filler material which, in one aspect, includes a gel material and water; and, in another aspect, includes a gel material, water and thermally conductive material or solids (e.g. but not limited to sand; powdered metal, e.g. aluminum, zinc, aluminum alloys, zinc alloys, iron, steel; and/or crushed granite). In particular aspects, either the gel/water or gel/water/solids mixture also includes a biocide and/or a cross-linking agent when the gel material is a polymer.
In certain aspects the gel material is a polymer, e.g. but not limited to xanthan gum, guar gum or polyacrylamide polymers; and in certain particular embodiments the resulting filler material does not crack when it shrinks and/or is rehydrateable. In other aspects the polymer is a rehydrateable polymer. Suitable gel materials include, but are not limited to xanthan biopolymers; commercially available Xanvis L(trademark) material from Kelco Oil Field Group; Kelzan L(trademark) material from Kelco Oil Field Group; ASP 700 polymer from Exxon-Nalco Co.; known drilling fluid polymers that form a gel with water; and synthetic polymers with suitable gelling characteristics; including, but not limited to, polyacrylamide polymers.
Such filler materials mentioned above may be used in any invention described below using grout, either instead of the grout or in combination with grout; and any grouting pipe and method described below may employ such filler material instead of and/or in addition to grout.
The present invention, in one embodiment, discloses a system for simultaneously installing a heat exchange fluid pipe loop and a grouting pipe in a wellbore. The system, in one embodiment, has a bottom member to which both pipes are attached and to which the grouting pipe is releasably attached. The bottom member may be of sufficient mass itself or it may have weights connected thereto so it will easily move down the wellbore. In another embodiment an integral loop of pipe is used with an inlet pipe secured to one side of the loop and an outlet pipe secured to the other side of the loop.
In one aspect the bottom member has an inlet connection and an outlet connection to which are secured inlet and outlet pipe of the pipe loop. A passageway through the bottom member provides for fluid communication between the inlet and outlet pipes so that heat exchange fluid may flow down the inlet pipe, through the passageway in the bottom member, and up through the outlet pipe.
In one aspect such a bottom member has a hole in which the grouting pipe is held. Pulling on the grouting pipe releases it from the bottom member for removal from the wellbore as grout flows out from the bottom of the grouting pipe.
In certain embodiments the grouting pipe is made of commercially available coiled tubing, e.g. in one aspect with an inside diameter of about one and three-tenths inches and an outside diameter of about one and a half inches; and the pipe loop is, e.g., three quarters of an inch in inside diameter made of high density polyethylene. In certain embodiments a wellbore for such heat exchange systems is three to three-and-a-half inches in diameter. In one aspect the bottom member is made of plastic and is pointed to facilitate its downward movement in the wellbore.
In one system and method according to the present invention a coiled tubing unit (CTU) is used to drill heat loop bore holes. The CTU has a reel on which is wrapped continuous flexible steel tubing, an injector which transports the tubing into and out of the hole, a drill bit on the end of a down hole motor, and a pump which supplies fluid for drilling. The motor is rotated by the pump pressure from the surface, which allows the unit to drill without rotating the drill string. This feature results in several benefits not possible with conventional drilling rigs. Directional drilling allows multiple wells to be drilled from one location. It also reduces the space required between bore holes and allows them to be drilled in a very close proximity to the subject building. This process not only reduces 80 percent of trenching on some jobs, but allows the unit to drill under existing slabs, driveways, parking lots and buildings. The compact design and directional drilling capabilities opens the retrofit market to geothermal systems.
With a method according to the present invention a relatively short surface trench is excavated before drilling is started. The drilling machine straddles the trench, drilling bore holes in the bottom of the trench as it moves over the length of the trench. A solids control system which cleans the drilling fluid as it is pumped from the hole, allowing cuttings to be dry discharged in a designated area, thereby maintaining a clean, dry drill site. As each hole is drilled, a track mounted rig moves approximately two to three feet down the trench to the next drilling location. A grout reel is then positioned over the previously drilled hole. This reel has a flexible grout pipe wrapped around a powered reel. As the grout pipe is pushed down the bore hole, it takes a plastic heat loop with it to the bottom of the hole. In certain preferred embodiments the loop is secured in the hole with an anchor apparatus; then the grout pipe is retracted while filling the hole with grout. Since a sinker bar is not required in this process, a 3 to 3xc2xc inch diameter hole is drilled, in certain embodiments, compared to a conventional 4 to 4xc2xd inch hole. This results in faster penetration, improved fuel efficiency, and improved heat transfer to the earth.
After installation of heat loops in multiple adjacent holes, the loops are heat fused into a common manifold. A return line to a facility or building is attached to the manifold and purged of all remaining air. The system is then pressure tested before being attached to a heat pump.
This invention provides these benefits: shorter surface trench and dry discharge results in less site damage; smaller bore hole increases system efficiency by improved heat transfer; total system installation time is reduced by at least 50 percent as compared to some prior art methods; and usable space is increased by drilling under slabs and other surface structures.
The U.S. Department of Energy states in a recent report that a system with these capabilities is needed to meet its goal of 400,000 installations by the year 2000.
In certain embodiments, the present invention discloses a system with coil tubing and a grout pipe with a curved member or members or a solid or hollow ball or partial ball at the end of the pipes to facilitate movement of the system through a wellbore and to prevent the lower end of the system from hanging up on or being caught by a ledge or uneven portion of the wellbore.
What follows are some of, but not all, the objects of this invention. In addition to the specific objects stated below for at least certain preferred embodiments of the invention, other objects and purposes will be readily apparent to one of skill in this art who has the benefit of this invention""s teachings and disclosures. It is, therefore, an object of at least certain preferred embodiments of the present invention to provide:
New, useful, unique, efficient, nonobvious devices and methods for systems and methods for installing heat exchange pipe loops in wellbores; filler materials for emplacement around such loops and methods employing such materials; for grouting and/or adding filler material to such wellbores; and for drilling such wellbores;
Such systems using a filler material that includes a thermally conductive material in suspension with a gel material such as a suitable polymer and, in one aspect, with a cross-linking agent to assist in maintaining solids of the thermally conductive material in suspension with the polymer;
Such systems including a bottom member to which a pipe loop and a filler or grouting pipe are secured, the bottom member for releasably holding the pipe, the bottom member for facilitating entry of the pipes into the wellbore, and, in one aspect, a curved member or members at the end of the pipes and tubing to facilitate movement of the system through a wellbore;
Such systems with a bottom member having a bore for receiving two pieces of a heat loop and coil tubing (or a connector for connecting coil tubing to the bottom member and, in one aspect, such a bottom member with a hole therethrough for pumping material through the coil tubing and out from the bottom member; and
Heat exchange systems with a plurality of heat exchange pipe loops drilled relatively close to each other with simultaneous filling and/or grouting of one wellbore while another wellbore is being drilled.
Certain embodiments of this invention are not limited to any particular individual feature disclosed here, but include combinations of them distinguished from the prior art in their structures and functions. Features of the invention have been broadly described so that the detailed descriptions that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. There are, of course, additional aspects of the invention described below and which may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention. The claims of this invention are to be read to include any legally equivalent devices or methods which do not depart from the spirit and scope of the present invention.
The present invention recognizes and addresses the previously-mentioned problems and long-felt needs and provides a solution to those problems and a satisfactory meeting of those needs in its various possible embodiments and equivalents thereof. To one of skill in this art who has the benefits of this invention""s realizations, teachings, disclosures, and suggestions, other purposes and advantages will be appreciated from the following description of preferred embodiments, given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. The detail in these descriptions is not intended to thwart this patent""s object to claim this invention no matter how others may later disguise it by variations in form or additions of further improvements.